1. Field of the Invention
The present invention relates to an optical system and a method of designing the optical system. More particularly, the present invention relates to an optical system and a method for designing the optical system in which chromatic aberration is favorably corrected by means of a combination of a diffractive optical element and a refractive optical element to easily achieve excellent optical performance. The present invention is suitably applied to optical apparatuses such as silver-halide film cameras, video cameras, electronic still cameras and digital cameras.
2. Related Background Art
As a method of correcting chromatic aberration of an optical system, there is a conventional method in which two glass materials (or lenses) with different dispersions are used in combination.
Besides this conventional method of reducing chromatic aberration by means of the combination of glass materials, there is a known method in which chromatic aberration is reduced by provision of a diffractive optical element(s) having a diffracting function on a lens surface or as a part of the optical system (see, for example, Japanese Patent Application Laid-Open No. H04-213421 (a counterpart of U.S. Pat. No. 5,044,706) and Japanese Patent Application Laid-Open No. H06-324262 (a counterpart of U.S. Pat. No. 5,790,321)).
This method utilizes the physical phenomenon that chromatic aberration with respect to rays of a certain standard wavelength is generated in the opposite directions between a refractive surface and a diffractive surface in an optical system. In addition, it is possible to allot an effect similar to that of an aspherical lens to such a diffractive optical element by changing the period of its periodic structure. This is very effective in reducing aberrations.
Since diffractive optical elements have a strong achromatizing effect and an effect similar to that of an aspherical surface, various optical systems in which a combination of a diffractive optical element(s) and a refractive optical element(s) are used for achromatization have been proposed. However, it has not been achieved to correct spherical aberration at a design standard wavelength and a color flare component such as chromatic spherical aberration excellently, while correcting longitudinal chromatic aberration and chromatic aberration of magnification.